1. Field of the Invention
Exemplary embodiments of the present invention relate to a hydraulic braking device and a method for controlling the same, and more particularly, to a hydraulic braking device which supplies braking pressure to a wheel cylinder according to the displacement of a pedal, and a method for controlling the same.
2. Description of the Related Art
In general, a braking device of a vehicle may include a vacuum brake to generate a braking force using suction pressure of an engine and a hydraulic brake to generate a braking force using hydraulic pressure.
A hybrid vehicle such as a fuel cell vehicle or electrical vehicle uses a hydraulic brake to implement an idle stop function for improving fuel efficiency, because the fuel cell vehicle or electrical vehicle has no engine.
An active hydraulic booster (AHB) is a kind of a hydraulic braking device which detects the displacement of a pedal through an electronic control unit (ECU) when a driver steps on the pedal, and supplies braking pressure to a wheel cylinder of each wheel so as to generate a braking force. The AHB is also referred to as an electro-hydraulic brake system.
As illustrated in FIG. 1, the electro-hydraulic brake system includes a reservoir 10, a hydraulic pressure supply unit, a main flow path, a pressurizing valve, a master cylinder 41, a sub path, and a shutoff valve. The reservoir 10 stores hydraulic oil therein. The hydraulic pressure supply unit sucks the hydraulic oil from the reservoir 10 and supplies braking pressure to a wheel cylinder. The main flow path supplies the braking pressure of the hydraulic pressure supply unit to the wheel cylinder. The pressurizing valve opens and closes the main flow path. The master cylinder 41 generates braking pressure. The sub path supplies the braking pressure of the master cylinder 41 to the main flow path. The shutoff valve opens and closes the sub flow path.
The ECU detects the displacement of a pedal, and transmits an opening/closing signal to the shutoff valve and the pressurizing valve according to the displacement of the pedal.
When a driver steps on the brake pedal during a normal braking operation, the ECU detects the displacement of the brake pedal through a pedal displacement detector 51, closes the shutoff valves 63 and 64 so as not to transfer hydraulic pressure from the master cylinder 41 to the wheel cylinder, calculates wheel pressure according to a pressure signal of a pressure sensor, and adjusts the pressure of each wheel through independent feedback control.
When electronic control is not performed, the shutoff valves are normally open. Thus, the hydraulic pressure of the master cylinder, generated by the driver, is introduced into the wheel cylinder to perform a braking operation at the minimum level when the system is broken down.
Such an electronic-hydraulic brake system has been disclosed in detail in Korean Patent Laid-open Publication No. 2013-0105960. Thus, the detailed descriptions thereof are omitted herein.
The electronic-hydraulic brake system determines whether to enter a brake control mode according to how much the driver steps on the pedal. Before the pedal position at which the brake control mode is started, a pedal operation section exists, in which the electronic-hydraulic brake system does not enter the brake control mode, but closes only the shutoff valves to form a closed hydraulic circuit.
When the driver steps on the pedal, the pedal reaches a brake control position through the pedal operation section in which the pressurizing valves are not opened but only the shutoff valves are closed.
FIG. 2 illustrates a flow of hydraulic pressure during brake control. During the brake control operation as illustrated in FIG. 2, the shutoff valves 63 and 64 are closed so as not to transfer high pressure from the hydraulic pressure supply unit to the driver through the pedal, and the pressurizing valves 34 and 35 are opened to transfer pressure to the wheel cylinder.
However, in the pedal operation section of the pedal as illustrated in FIG. 4, the pressurizing valves 34 and 35 are not opened before the braking pressure supplied to the wheel cylinder from the hydraulic pressure supply unit is formed. Thus, the flow path between the shutoff valves 63 and 64 and the pressurizing valves 34 and 35 may form a closed hydraulic circuit. Such a situation may occur when the driver steps on the pedal or the value of the pedal displacement detector has an offset. This is because the path of the master cylinder connected to the pedal from the hydraulic circuit is blocked before entering the brake control mode.
The pressurizing valve normally blocks hydraulic oil stored at high pressure in an accumulator, and passes the hydraulic oil only during the brake control, thereby forming braking pressure.
At this time, the pressurizing valve may not completely block the high pressure due to various factors, but a leak may occur. When the leak occurs, the braking pressure may be transferred to the wheel cylinder, and cause a drag which is not required for the vehicle. That is, when a leak occurs in the pressurizing valve while the closed hydraulic circuit of the pedal operation section is formed, the pressure of the closed hydraulic circuit may be increased to generate braking pressure. Then, the braking pressure may cause a drag.
Conventionally, when the position of the pedal is continuously maintained without deviating from the pedal operation section, a drag could not be prevented from occurring.